Conventionally, a two-dimensional image detecting device for radiation has been known in which semiconductor sensors for detecting an X-ray and a generating electrical charge(electron-positive hole) are two-dimensionally disposed, each sensor is provided with an electrical switch, and the electrical switches are successively turned on for each line and electrical charge is read for each raw.
A specific structure and principle of such a two-dimensional image detecting device are described in "D. L. Lee, et al., `A New Digital Detector for Projection Radiography`, Proc. SPIE, Vol. 2432, Physics of Medical Imaging, pp. 237-249, 1995", "L. S. Jeromin, et al., `Application of a-Si Active-Matrix Technology in a X-ray Detector Panel`, SID(Society for Information Display) International Symposium, Digest of Technical Papers, pp. 91-94, 1997", and Japanese Laid-Open Patent Publication No.342098/1994 (Tokukaihei 6-342098).
The following explanation describes the specific structure and principle of the conventional two-dimensional image detecting device for radiation. FIG. 12 is a perspective view showing a model of the construction of the two-dimensional image detecting device for radiation. Further, FIG. 13 is a sectional view showing a model of the structure for one pixel.
As shown in FIGS. 12 and 13, the two-dimensional image detecting device for radiation is provided with an active-matrix substrate having electrode wires(gate electrode 52 and source electrode 53), TFT(thin film transistor)54 and electrical charge storage capacity(Cs)55 arranged in an XY matrix form on a glass substrate 51. Moreover, a photoconductive film 56, a dielectric layer 57, and an upper electrode 58 are formed on virtually the entire surface of the active-matrix substrate.
The electrical charge storage capacity 55 has a construction in which a Cs electrode 59 opposes to a pixel electrode 60 connected with a drain electrode of the TFT 54 via an insulating film 61.
For photoconductive film 56, semiconductive materials are used so as to generate electrical charge by exposure to radiation such as an X-ray. According to the aforementioned reference books, amorphous selenium(a-Se), which has high dark resistance and favorable photoconductivity, has been used. The photoconductive film 56 is formed with a thickness of 300.about.600 .mu.m by using a vacuum evaporation method.
Further, an active-matrix substrate, which is formed in a manufacturing process of a liquid crystal display device, can be applied to the aforementioned active-matrix substrate. For example, the active-matrix substrate used for an active matrix liquid crystal display device(AMLCD) is provided with the TFT made of amorphous silicon(a-Si) or poly-silicon(P-Si), an XY matrix electrode, and electrical charge storage capacity. Therefore, only a few changes in arrangement make it easy to use the active-matrix substrate as that of the two-dimensional image detecting device for radiation.
The following explanation describes a principle of operations of the two-dimensional image detecting device for radiation having the above-mentioned structure.
Electrical charge is generated when the photoconductive film 56 is exposed to radiation. As shown FIGS. 12 and 13, the photoconductive film 56 and the electrical charge storage capacity 55 are electrically connected in series with each other; thus, when voltage is applied between the upper electrode 58 and the Cs electrode 59, electrical charge generated in the photoconductive film 56 moves to a positive electrode side and a negative electrode side. As a result, the electrical charge storage capacity 55 stores electrical charge. Further, an electron blocking layer 62 made of a thin insulating layer is formed between the photoconductive film 56 and the electrical charge storage capacity 55. The electron blocking layer 62 acts as a blocking photodiode for preventing electrical charge from being injected from one side.
With the above-mentioned effect, the TFT 54 comes into an open state in response to input signals of gate electrode G1, G2, G3, . . . , and Gn so that the electrical charge stored in the electrical charge storage capacity 55 can be applied to the outside from source electrodes S1, S2, S3, . . . , and Sn. The gate electrodes 52, the source electrodes 53, the TFT 54, and the electrical charge storage capacity 55, etc. are all formed in a matrix form; therefore, it is possible to two-dimensionally obtain image information of an X-ray by scanning signals for each line inputted to gate electrodes G1, G2, G3, . . . , and Gn.
Additionally, in the case when the photoconductive film 56 has photoconductivity for a visible ray and an infrared ray as well as for the radiation such as an X-ray, the above-mentioned two-dimensional image detecting device acts as a two-dimensional image detecting device for detecting the visible ray and the infrared ray.
However, the conventional arrangement has used a-Se as the photoconductive film 56. Since the a-Se has dispersive conductivity of photoelectric current, that is peculiar to amorphous materials, the a-Se is inferior in response and the sensitivity(S/N ratio) to an X-ray is not sufficient. Therefore, the electrical charge storage capacity 55 needs to be fully charged by being exposed to the X-ray for a long time in order to read information.
Further, upon irradiation of X-ray, in order to prevent electrical charge from being stored in the electrical charge storage capacity due to leakage current and in order to reduce leakage current(dark current), the dielectric layer 57 is provided between the photoconductive film 56 and the upper electrode 58. Since it is necessary to add a step(sequence) for removing electrical charge remained in the dielectric layer 57 for each frame, the above-mentioned two-dimensional image detecting device is available only when photographing a static picture.
In response to this problem, in order to obtain image data corresponding to a moving image, it is necessary to use the photoconductive film 56 which is made of crystal(or polycrystal) material and is superior in X-ray sensitivity(S/N ratio). If the sensitivity of the photoconductive film 56 improves, it becomes possible to sufficiently charge the electrical charge storage capacity 55 even when X-ray is applied for a short time. Further, the need for applying high voltage to the photoconductive film 56 is eliminated; thus, it is not necessary to arrange dielectric layer 57.
As photoconductive materials which are superior in X-ray sensitivity, CdTe and CdZnTe have been known. Generally, photoelectricity absorption for X-ray proportionally increases to the effective atomic number of absorbed substance that is multiplied to the fifth power. For example, if it is assumed that the atomic number of Se is 34 and the effective atomic number of CdTe is 50, the sensitivity is expected to improve by approximately 6.9 times. However, in the case when CdTe or CdZnTe is adopted instead of a-Se as a material of the photoconductive film 56 of the two-dimensional image detecting device for radiation, the following problem arises:
In the case of the conventional a-Se, a vacuum evaporation method can be adopted as a film-forming method and a film can be formed at a normal temperature; thus, it has been easy to form a film on the active-matrix substrate. Meanwhile, in the case of CdTe and CdZnTe, film-forming methods such as an MBE(molecular beam epitaxy)method and an MOCVD(metal organic chemical vapor deposition)method have been known. Especially when forming a film on a large substrate is taken into consideration, it is understood that the MOCVD method is suitable.
However, in the case when a material selected from CdTe and CdZnTe is made into a film by using the MOCVD method, the heat decomposition temperature of organic cadmium(dimethyl cadmium(DMCd)) is approximately 300.degree. C., and the respective heat deposition temperatures of organic tellurium(diethyl tellurium(DETe) and diisopropyl tellurium(DiPTe)) are approximately 400.degree. C. and 350.degree. C.; therefore, forming a film requires a high temperature of approximately 400.degree. C.
Generally, in the TFT 54 which is formed on the active-matrix substrate, an a-Si film and a p-Si film are used as a semiconductive layer, and these films are formed at a film-forming temperature of 300.about.350.degree. C. while hydrogen(H.sub.2) being added in order to improve the semiconductive property. The TFT element formed in such a process has a heat-resistance temperature of approximately 300.degree. C.; thus, if the TFT element is processed at a temperature exceeding the heat-resistance temperature, hydrogen is released from the a-Si film and the p-Si film; consequently, the conductive property is degraded.
Therefore, it has been practically difficult to make a material selected from CdTe and CdZnTe into a film on the active-matrix substrate by using the MOCVD method from the perspective of the film-forming temperature.